Blade tip clearance measurement sensor for gas turbine engines

ABSTRACT

An electromagnetic field sensor assembly for blade tip clearance measurement in a gas turbine engine is disclosed that includes a ceramic sensor body, a multi-layered wire coil wound about a distal end portion of the sensor body for producing an electromagnetic field, a ceramic well enclosing the sensor body and the coil, and a metallic housing surrounding the well and having an open distal end.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject application is a continuation of U.S. application Ser. No.12/286,262 filed Sep. 29, 2008 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention is directed to electromagnetic field sensors, andmore particularly, to a blade tip measurement system that uses amarginal oscillator circuit and a heat resistant sensor assembly togenerate an RF electromagnetic field in a casing of a gas turbineengine, whereby perturbation of the field by an array of rotating bladetips represents a change in susceptibility that impacts the circuit toindicate a clearance measurement between the blade tips and the enginecasing.

2. Description of Related Art

In an axial flow gas turbine engine, it is desirable to minimize theclearance between the blade tips of a turbine rotor and the enginecasing that surrounds the rotor. This is because excessive clearancesbetween the blade tips and the engine casing reduce engine efficiency,and contact between the blade tips and engine casing causes damage tothe engine.

The prior art includes a variety of solutions for maintaining blade tipclearance in a gas turbine engine. One such solution is a mechanicalsystem for adjusting the radial position of the casing surrounding therotor blades to improve engine efficiency, as disclosed in U.S. Pat. No.5,104,287. Another solution is a mechanical system for moving the rotordisc relative to the engine casing, as disclosed for example in U.S.Pat. No. 5,330,320. An active clearance control systems that includes anactuator for moving a rotor blade assembly relative to the engine casingto maintain the minimum design clearance between the blade tips and theengine casing is disclosed in U.S. Pat. No. 7,407,369, the disclosure ofwhich is incorporated by reference in its entirety.

These clearance control systems require a mechanism for monitoring bladetip clearance with a high degree of accuracy, under the severeenvironmental conditions that exist within the turbine gas path. Thesesevere conditions include high blade tip speeds, vibration modes, highpressure fluctuations and the exceedingly high temperatures of theturbine gases, which can be as hot as 1400° C.

An example of a prior art blade tip clearance monitoring system isdisclosed in U.S. Pat. No. 6,678,060 to Hayworth, which employs a groupof photo-cells that monitor the position of the blade tips by detectingchanges in the shape of a detected image. Another example of a prior artblade tip monitoring system is disclosed in U.S. Pat. No. 5,739,524 toFally, which employs an optical probe that senses the distance of anobject by measuring reflected radiation.

While optical measuring devices known in the art are effective formeasuring blade tip clearance, they are often susceptible to the severethermal environment of a gas turbine engine, leading to difficulties incalibration, which can result in inaccurate measurements over time.

SUMMARY OF THE INVENTION

The subject invention is directed to an electromagnetic field sensorassembly, and more particularly, to a sensor assembly and system formonitoring or otherwise measuring blade tip clearance in a gas turbineengine. The sensor assembly is particularly well adapted for the hightemperature operating environment that exists within a gas turbineengine. In this regard, the sensor assembly includes a ceramic sensorbody, a wire coil wound about a distal end portion of the sensor bodyfor producing an electromagnetic field, a ceramic well enclosing thesensor body and the coil, and a metallic outer housing surrounding aperiphery of the ceramic well and having an open distal end. The outerhousing of the sensor assembly is configured to be mounted in the enginecasing adjacent an array of blade tips.

The sensor body is preferably formed from aluminum oxide or a similarrefractory material. The wire coil wrapped about the distal end portionof the sensor body is preferably formed from wire comprised of aplatinum group metal or an alloy thereof. For example, the wire may beformed from Pt-10Rh, which is a platinum alloy that includes 10%Rhodium. Alternatively, the wire may be formed from an oxide dispersionstrengthened platinum group metal or alloy thereof. Preferably, the coilis formed from ceramic coated wire, and more preferably, the wireforming the coil is coated with aluminum oxide. The coil is preferablyformed in plural layers, with each layer having a plurality of turns.Preferably, the distal end portion of the sensor body has an annularrecess for accommodating the multi-layered coil, and the coil isanchored to the sensor body within the annular recess by cement.

The ceramic well is a sealed enclosure and is preferably formed fromaluminum oxide, and it has a metallized section to facilitate attachmentto the metallic outer housing. Preferably, the metallic outer housing isformed from a heat resistant metal that is compatible with the materialfrom which the adjacent engine casing is constructed, such as, forexample, Ni—Cr alloy 600. It is envisioned that the outer housing mayinclude a plurality of longitudinally extending cooling channelscommunicating with the exterior of the engine casing.

Preferably, a transition member is provided for joining the ceramic wellto the metal housing, while serving to buffer thermal stress in thesensor assembly. A cable adapter is also joined to the metallic housingadjacent the proximal end portion thereof, and a pair of coaxial cableassemblies are joined to the cable adapter for connection with the coilof the sensor body. More particularly, cable lead wires join the centerconductors of the coaxial cables to opposed ends of the coil. In anembodiment of the subject invention, the center conductor of eachcoaxial cable is formed from two different materials including a firsttemperature resistant material located adjacent to the sensor assemblyand a second material located remote from the sensor assembly in aregion of lower temperature.

In another embodiment of the subject invention, the sensor assemblyincludes two sensor bodies each having a multi-layered coil associatedwith the distal end portion thereof, wherein each coil is driven by orotherwise forms part of a separate marginal oscillator circuit. In suchan instance, the distal end portions of the two sensor bodies areaxially off-set from one another to facilitate system levelself-calibration.

The subject invention is also directed to a blade tip clearancemeasurement system for a gas turbine engine that includes anelectromagnetic field sensor assembly positioned in the casing of a gasturbine engine adjacent an array of blade tips, a marginal oscillatorcircuit operatively connected to the sensor assembly for generating anelectromagnetic field in relation to the blade tips, and means forprocessing an output signal received from the sensor assembly inresponse to perturbation of the electromagnetic field by passage of theblade tips therethrough, wherein the output signal is indicative of theposition of the blade tips relative to the engine casing. In essence,the coil wire that is positioned within the temperature resistant sensorassembly is a remote extension of the marginal oscillator circuit.

The subject invention is further directed to a method for measuringblade tip clearance in a gas turbine engine that includes the steps ofgenerating an electromagnetic field between the engine casing and anarray of rotating blade tips, sensing changes in the electromagneticfield as the rotating blade tips pass therethrough, and determining theposition of the blade tips relative to the engine casing based uponchanges in the electromagnetic field.

These and other features of the blade tip clearance measurement system,sensor assembly and measurement method of the subject invention willbecome more readily apparent to those having ordinary skill in the artfrom the following detailed description of the invention taken inconjunction with the several drawings described below.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject inventionappertains will readily understand how to make and use the blade tipclearance sensor, sensing system and sensing method of the subjectinvention without undue experimentation, preferred embodiments thereofwill be described in detail below with reference to certain figures,wherein:

FIG. 1 is a perspective view of an electromagnetic sensor assembly formeasuring blade tip clearance in a gas turbine engine, constructed inaccordance with a preferred embodiment of the subject invention, whereinthe sensor assembly is mounted in the engine casing of a gas turbineengine adjacent the outer periphery of a rotor disc carrying an array ofturbine blades, and is operatively associated with a marginal oscillatorcircuit for generating a RF electromagnetic field and a signalprocessing circuit for processing output signals from the sensorassembly in response to perturbation of the field by the blade tipsduring engine operation;

FIG. 2 is a perspective view of the electromagnetic sensor assembly ofthe subject invention;

FIG. 3 is an exploded perspective view of the electromagnetic sensorassembly of shown in FIG. 2, with parts separated for ease ofillustration;

FIG. 4 is an exploded perspective view of the ceramic well and sensorbody of the electromagnetic sensor assembly of FIGS. 2 and 3, showingthe wound coil in a recess formed at the distal end portion of theceramic body;

FIG. 5 is cross-sectional view, taken along line 5-5 of FIG. 1,illustrating the electromagnetic sensor assembly during engineoperation, wherein a RF electromagnetic field is generated to measure orotherwise monitor the clearance between engine casing and the bladetips;

FIG. 5 a is a localized view of the distal end portion of the sensorassembly, illustrating the multi-layered wrapping of the coil turns;

FIGS. 6 and 7 are cross-sectional views of the proximal end portion ofthe sensor assembly of the subject invention, illustrating the steps offilling the sensor cavity with a ceramic powder or felt and thenintroducing an inert gas into the cavity to protect the cable lead wiresfrom oxidation;

FIG. 8 is a perspective view of the electromagnetic sensor assembly ofthe subject invention, which employs coaxial conductors that include twodifferent materials to form the center conductors, as shown illustratedFIG. 9;

FIG. 10 is and exploded perspective view of another embodiment of theelectromagnetic sensor assembly of the subject invention, which includestwo sensor bodies; and

FIG. 11 is a cross-sectional view of the electromagnetic sensor assemblyof FIG. 10, illustrating the axially off-set sensor bodies locatedwithin the sensor assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is illustrated a blade tip clearancemeasurement sensor assembly constructed in accordance with a preferredembodiment of the subject invention and designated generally byreference numeral 10. As illustrated, sensor assembly 10 is mounted inthe casing 12 of a gas turbine engine adjacent to a blade tip 14 of aturbine blade 16. While not shown, those having ordinary skill in theart will readily appreciate that an array of turbine blades 16 aremounted on a rotating disc within the engine casing 12.

Sensor assembly 10 employs a RF electromagnetic field to measure orotherwise monitor the clearance that exists between the blade tips 14and the engine casing 12, with a high degree of accuracy. As explainedin more detail below, sensor assembly 10 is constructed in such a mannerso as to withstand the severe environmental and operational conditionsthat exist in and adjacent to the turbine gas path, where operatingtemperatures often reach as high as 1400° C.

As shown schematically in FIG. 1, the sensor assembly 10 is operativelyassociated with a marginal oscillator circuit 20. The marginaloscillator circuit 20 is designed to generate a RF electromagnetic fieldfor measuring blade tip clearance, in near real time. An example of sucha circuit is disclosed in U.S. Pat. No. 6,984,994, the disclosure ofwhich is incorporated herein by reference in its entirety.

During engine operation, perturbation of the RF electromagnetic field bythe blade tips 14 moving therethrough represents a change insusceptibility that impacts the marginal oscillator circuit 20,providing a signal indicative of the position of the blade tips 14relative to the engine casing 12. In this regard, the storage of energyby the field gives rise to a change in frequency (FM), while a loss ofenergy from the field gives rise to a change in its amplitude (AM). Bothof these signatures are part of the output signal data stream. In otherwords, the sensor assembly 10 supports two simultaneous data streamsrelating to blade tip clearance, including one data stream that isrelated to oscillator frequency and one data stream that is related tooscillator amplitude.

A signal processing circuit 22 conditions the output signal from thesensor assembly 10 and oscillator 20 to provide an output measurement ofthe blade tip clearance that is readily interpreted by those havingordinary skill in the art. Moreover, the output signal from the sensorassembly 10 and oscillator 20 is conditioned by the signal processor 22in such a manner so as to provide an indication of the gap distance thatexists between the rotating blade tips 14 and the engine casing 12, asshown for example in FIG. 5.

Referring now to FIG. 2, the sensor assembly 10 of the subject inventionincludes a cylindrical outer housing 24 that is adapted and configuredto mount to the casing 12 of a gas turbine engine, as shown in FIGS. 1and 5. The outer housing 24 is formed form a heat resistant metal thatis preferably compatible with the material from which the surroundingengine casing is constructed, such as, for example, Ni—Cr alloy 600 or asimilar material. The outer housing 24 of sensor assembly 10 has an opendistal end, as shown in FIG. 2, and a proximal mounting flange 26dimensioned to be positioned against the exterior surface of the enginecasing 12.

As best seen in FIG. 5, the open distal end of the outer housing 24 isslightly recessed from the interior surface of the engine casing 12, toprotect the sensor assembly 10 from damage during engine operation.Because of this special accommodation, the tip clearance sensing systemmust be calibrated by correcting the output signal from the sensorassembly to account for the additional distance between the enginecasing 12 and the electromagnetic field sensing element.

A plurality of circumferentially spaced apart, longitudinally extendingcooling channels 28 are formed in the exterior surface of outer housing24. The cooling channels 28 direct cool air toward the distal sensorhead located within the engine casing as a result of the pressuredifferential that exists between the exterior of the engine casing wherethe flange 26 is located and the interior of the engine casing where thedistal sensing head of the sensor assembly is located. Those skilled inthe art will readily appreciate that these cooling channels would onlybe effective in engine applications where the hot side of the enginecasing is at a lower pressure than the cool side of the engine casing.In engine applications where this pressure differential is not present,the cooling channels in the outer housing can be eliminated.

With continuing reference to FIG. 2, the sensor assembly 10 furtherincludes a cylindrical ceramic well 30, which contains or otherwiseencloses the internal sensor components, as described in more detailbelow with respect to FIGS. 3 and 4. Sensor assembly 10 also includes ametal cable adapter 32 having a flange 25 that is welded to the mountingflange 26 at proximal end of the outer housing 24. Cable adapter 32facilitates the connection of a pair of metal-sheathed coaxial cables 34and 36 with the sensor assembly 10. The metal sheathed coaxial cables 34and 36 are attached to the metal cable adapter 32 by brazing or asimilar mechanical joining technique.

Coaxial cables 34 and 36 operatively connect the sensor assembly 10 tothe marginal oscillator circuit 20 and the signal processing circuit 22,respectively. The coaxial cables 34 and 36 are low-loss ceramicinsulated, metal sheathed, RF cables (e.g., silicon oxide insulated RFcables or the like) that are adapted to minimize high frequency losses.In accordance with the subject invention, signals are transmitted to andfrom sensor assembly 10 by way of the center conductors of coaxialcables 34 and 36. The outer conductors provide dielectric shielding forthe center conductors and form part of the hermetically sealed closurewhich defines the sensor assembly 10.

Referring now to FIGS. 3 and 4, the ceramic well 30 of sensor assembly10 is formed from aluminum oxide and is secured to the outer housing 24by a metallic transition ring 38. More particularly, a metallizedcoating indicated by reference numeral 35, is applied to the proximalend portion of the ceramic well 30. An example of a suitable metallizedcoating consists of a molybdenum manganese film with nickel plating. Anannular section of the inner periphery of the transition ring 38 isbrazed or otherwise mechanically joined to the metallized surface 35 ofthe ceramic well 30, while an annular section of the outer periphery ofthe transition ring 38 is brazed or otherwise mechanically joined to aninner annular section 27 of the outer metallic housing 24, belowmounting flange 26, as best seen in FIG. 5. The transition ring 38 ispreferably formed from Pt-10Rh and serves to buffer thermal stressbetween the ceramic well 30 and the outer metal housing 24.

As best seen in FIG. 4, sensor assembly 10 further includes acylindrical sensor body 40 or former supported within the ceramic well30. Sensor body 40 is formed from a ceramic material, such as forexample, aluminum oxide or a similar refractory material. An annularrecess 42 is formed in the distal end portion of the sensor body foraccommodating a wound wire coil 44. Wire coil 44 serves as thefunctional sensing element of sensor assembly 10 by producing the RFelectromagnetic field generated by the marginal oscillator circuit 20 tomeasure the clearance between the engine casing 12 and blade tips 14, asshown for example in FIG. 5. In essence, the wire coil 44 is a remoteextension of the marginal oscillator circuit 20 located adjacent to thetarget or object that is to be sensed, namely, the blade tips 14.

As explained above, the distal end portion of the sensor assembly 10,and hence the coil wire 44, is recessed into the engine casing wall toprotect the sensor assembly 10 during engine operation. Since thedistance between the coil 44 and engine casing 12 is a fixed distance,the output signal from the coil 44 can be corrected as part of thesystem calibration to provide the requisite clearance distance betweenthe engine casing 12 and the blade tips 14.

The coil 44 is formed from a heat resistant wire comprised of a platinumgroup metal or alloy thereof, such as, for example, Pt-10Rh, which is aplatinum alloy that includes 10% Rhodium. The wire of coil 44 is ofrelatively small gauge, such as, for example 0.003 inches, and ispreferably coated with ceramic, for example, aluminum oxide to provideelectrical insulation for the densely packed, multi-layered constructionof wire coil 44.

More particularly, as best seen in FIG. 5 a, to fit as much wire aspossible into the distal annular recess 42 of sensor body 40, the coil44 is formed in plural winding layers, with each winding layer havingmultiple turns. In an exemplary embodiment of the sensor assembly 10,the radially inner wound layer has 8 turns, the middle wound layer has 7turns and the radially outer wound layer has six turns. The coil 44 iscoated with cement (e.g. aluminum oxide and colloidal silica) to anchorthe wires to the sensor body 40.

The sensor body 40 also includes diametrically opposed lateral channels46 and 48 for respectively accommodating the lead wires 34 a and 36 athat extend from the two coaxial cable 34 and 36. Preferably, the leadwires 34 a and 36 a are fused to the center conductors of the coaxialcable 34 and 36. The lead wires 34 a and 36 a are also connected to theopposite ends of wire coil 44, within in diametrically opposed moldedrecesses 50, one of which is shown for example in FIG. 4.

In this regard, the lead wires 34 a and 36 a are fed from the respectivelateral channels 46 and 48, though internal passages formed within theceramic sensor body 40, and out to the opposed molded recesses 50 wherethey join the opposed ends of wire coil 44. The lead wires 34 a and 36 aare attached to the opposed ends of the wire coil 44 by a mechanicalcrimp or fuse joint or by similar mechanical means. Preferably, the leadwires 34 a and 36 a are formed from Pt-10Rh.

Alternatively, the lead wires 34 a and 36 a and the wire coil 44 can bemade from an oxide dispersion strengthened platinum group metal or alloythereof to enhance the high temperature reliability of the sensorassembly 10. This material is produced with fine ceramic particles, suchas zirconia or yittria, dispersed throughout the metal, that serve tostabilize the grain structure when used at high temperatures. In thefield of thermometry, it is known to improve the durability of sensorcomponents in this manner. An example of heat resistant wires for use intemperature sensing is disclosed in U.S. Pat. No 7,026,908, thedisclosure of which is incorporated herein by reference in its entirety.

As best seen in FIGS. 3 and 5, the proximal end portions of lead wires34 a and 36 a of cable adapter 32, which are located within the interiorcavity 45, are bent at a right angle. This structural geometry providesa means of thermal strain relief in the lead wires to ensure that areliable mechanical connection is maintained between the lead wires andthe opposed ends of coil wire 44.

Referring to FIG. 5, the ceramic sensor body 40 is mounted in orotherwise molded into place within the ceramic well 30 of sensorassembly 10 by a ceramic-based potting material 60. The potting material60 ensures that the sensor body 40 is securely positioned within thewell 30 so that the location of the coil 44 is rigidly maintained. It isenvisioned and within the scope of the subject disclosure that theceramic well and ceramic sensor body of sensor assembly 10 could beformed as a unitary ceramic component, rather than two separate elementsthat are joined together to form an integral structure. Those skilled inthe art will readily appreciate that the use of a platinum group metalor alloy thereof for forming the lead wires and wire coil isparticularly advantageous during fabrication of the sensor assembly,since that material can withstand the sintering temperatures that arerequired to fabricate a unitary ceramic component.

With continuing reference to FIG. 5, the inner diameter of the metalouter housing 24 has a reduced inner diameter, forming an annular recess55 that surrounds the distal portion of the ceramic well 30, in theregion of coil 44. This is done to minimize the influence of the metalhousing 24 on the electromagnetic field produced the coil 44.

As shown in FIG. 5, a cover 64 is provided at the proximal end of cableadapter 32 to enclose and seal the interior cavity 45 thereof. However,before welding the cover 64 to the cable adapter 32, the interior cavity45 of the cable adapter 32 is filled with a ceramic powder or felt, asshown for example in FIG. 6. Thereafter, the cover 64 is welded onto theadapter 32. Then, an inert gas is introduced into the cavity 35 througha port 66 in the cover 64. The port 66 is then closed with a welded plug68. The ceramic powder or felt provide mechanical support for the cablelead wires 34 a and 36 a, while the inert gas serves to protect thecable lead wires 34 a and 36 a from oxidation.

In an alternative embodiment of the subject invention, as shown forexample in FIGS. 8 and 9, a more robust cable configuration can beemployed. That is, the center conductor of each coaxial cable 34, 36would be formed from two different conductors 72, 74, consisting of twodifferent materials, materials joined together in series at a junctionpoint 75. The two conductor materials forming the center conductor ofthe coaxial cables would include a first temperature resistant conductormaterial, such as Pt-10Rh or a similar platinum based alloy, located ina region “a” of relatively high adjacent (e.g., 900° C.) to the sensorassembly 10 and a second conductor material, such as copper, located ina region “b” of relatively lower temperature (e.g., 250° C.) remote fromthe sensor assembly 10. This serial conductor configuration wouldeliminate the need for backfilling the interior cavity 45 of cableadapter cavity 32 with a protective inert gas.

Referring to FIG. 5, during engine operation, to monitor the clearance“x” that exists between the engine casing 12 and blade tips 14, in aneffort to maintain a minimum clearance, the coil 44 at the distal end ofsensor body 40 produces a RF electromagnetic field, which is generatedby the marginal oscillator circuit 20. The sensor assembly 10 detectschanges in the electromagnetic field as the rotating blade tips 14 passtherethrough. The signal processing circuit 22 then determines theposition of the blade tips 14 relative to the engine casing 12 basedupon changes in the electromagnetic field produced by the coil 44. Usingthat information, adjustments can be made to the rotor disc and/orengine casing, to minimize the clearance between the blade tips andengine casing, and thus improve engine efficiency.

Referring to FIGS. 10 and 11, there is illustrated another embodiment ofthe electromagnetic sensor assembly of the subject invention which isdesignated generally by reference numeral 100. Sensor assembly 100includes two sensor bodies 140 a and 140 b mounted side-by-side within asingle ceramic well 130 having parallel well chambers 130 a, 103 b. Theceramic well 130 is enclosed within a metal outer housing 124, with eachsensor body 140 a, 140 b having a separate wire coil 144 a, 144 b forproducing an independent electromagnetic field. That is, each coil isdriven by or otherwise a remote part of a separate marginal oscillatorcircuit.

As best seen in FIG. 11, the distal end portions of the sensor bodies140 a, 140 b and thus the coils 144 a and 144 b associated therewith areaxially off-set from one another. For example, one coil may be axiallydisplaced from the other coil by about 0.5 mm. As a result, each coilwill produce a different voltage and frequency response as the blade tippasses through the electromagnetic field. This dual coil arrangementwill facilitate system level self-calibration of the sensor system.

While the heat resistant electromagnetic sensor assembly and sensingsystem of the subject invention has been shown and described withreference to preferred embodiments, those skilled in the art willreadily appreciate that various changes and/or modifications may be madethereto without departing from the spirit and/or scope of the subjectdisclosure.

What is claimed is:
 1. An electromagnetic field sensor assembly,comprising: a) a ceramic sensor body; b) a wire coil wound about adistal end portion of the ceramic sensor body for producing anelectromagnetic field, wherein the wire coil is formed from a platinumgroup metal or an alloy thereof; c) a ceramic well enclosing the ceramicsensor body and the wire coil; d) a metallic housing surrounding aperiphery of the ceramic well and having an open distal end; e) a cableadapter joined to a proximal end portion of the metallic housing,wherein a pair of coaxial cable assemblies are joined to the cableadapter for connection with the coil, wherein cable lead wires arejoined to center conductors of the coaxial cables for connection toopposite ends of the coil, and wherein the center conductor of eachcoaxial cable is formed from two different conductor materials connectedin series to one another.
 2. An electromagnetic field sensor assemblycomprising: a) two ceramic sensor bodies; b) a wire coil wound about adistal end portion of each ceramic sensor body for producing anelectromagnetic field, wherein the wire coils are formed from a platinumgroup metal or an alloy thereof; c) a ceramic well enclosing the twoceramic sensor bodies and the wire coils; and d) a metallic housingsurrounding a periphery of the ceramic well and having an open distalend.
 3. An electromagnetic field sensor assembly comprising: a) twoceramic sensor bodies; b) a wire coil wound about a distal end portionof each ceramic sensor body for producing an electromagnetic field,wherein the wire coils are formed from a platinum group metal or analloy thereof; c) a ceramic well enclosing the two ceramic sensor bodiesand the wire coils; and d) a metallic housing surrounding a periphery ofthe ceramic well and having an open distal end, wherein the distal endportions of the two sensor bodies are axially off-set from one another.